1. Field of the Invention
The present invention relates to a production method of an optical waveguide to be widely used for optical communications, optical information processing, position sensors, and other general optics.
2. Description of the Related Art
Optical waveguides typically include a core provided in a predetermined pattern as a light path on a surface of an under-cladding layer, and an over-cladding layer covering the core. For improvement of mass-productivity, the optical waveguides are produced by a roll-to-roll process, which employs a metal substrate such as of stainless steel (SUS) capable of bearing a stress (cure shrinkage) occurring when a resin material is cured.
The SUS is a less expensive material substantially free from warpage which may otherwise occur due to a shrinkage stress of a coating film. On the other hand, the SUS is a metal that has a rough surface having minute irregularities and, therefore, is liable to cause irregular light reflection during an exposure process in formation of the core. This results in roughened wall surfaces of the core. To cope with this, it is proposed, for example, to blend a UV absorber in an under-cladding layer material for suppressing the back reflection of the metal substrate such as of the SUS (see JP-A-2009-276724).
For production of an optical touch panel including the optical waveguide of this type, it is necessary to employ an imprint process in order to impart a light output portion and a light input portion of the over-cladding layer with a lens shape. At this time, the steps of preliminarily forming an alignment mark on the core, positioning the core and the over-cladding layer with respect to each other while recognizing the alignment mark by means of an alignment camera, and bonding the core and the over-cladding layer together are essential for improvement of the accuracy of the positioning of a light output end of the core and the over-cladding layer (lens) (see JP-A-2008-203431).
In the conventional optical waveguide production process, a light source capable of emitting visible light (in a wavelength range of about 400 to about 700 nm) is used for the alignment camera, and the SUS is typically used for the metal substrate. Therefore, the alignment camera is liable to pick up an image of the rough surface of the SUS. This, for example, reduces the contrast of an edge of the alignment mark provided on the core, resulting in disadvantageous reduction in the accuracy of the detection of the alignment mark by the alignment camera. To cope with this disadvantage, it is proposed, for example, to form an alignment mark having a rounded edge by exposure with a halftone mask in the alignment mark forming step to increase the contrast of the alignment mark for improvement of the degree of the recognition. However, the formation of the alignment mark by the exposure method using the halftone mask leads to lower finishing accuracy. Therefore, this method is not satisfactory for the improvement of the recognition of the alignment mark, reducing the production efficiency (yield).